In a case where a multilayer optical disc is played, for example, there is a high possibility that the light amount of a signal reduces to cause an error in reading the signal. As a solution to this problem, there is known a homodyne detection method by which to amplify the detection signal by the use of light interference (refer to Patent Documents 1 and 2).
According to Patent Documents 1 and 2, as a homodyne method for detecting light resulting from interference between signal light and reference light, four sets of signal light and reference light with phase differences of 90 degrees are detected. Specifically, the sets of signal light and reference light with phase differences of 0, 90, 180, and 270 degrees are detected. Each of the detections is performed by detecting the intensity of light resulting from interference between the signal light and the reference light.
Further, Patent Document 3 describes a playing apparatus that uses the homodyne method for optical discs on which signals are recorded in both lands and grooves.